Thermal spraying powder, thermal spraying method, and method for forming thermal spray coating

ABSTRACT

A thermal spraying powder contains a chromium-iron based alloy powder that includes carbon. The ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 2% or more. 10% particle size D 10  of the alloy powder is preferably 10 μm or more, and more preferably 15 μm or more. 50% particle size D 50  of the alloy powder is preferably 20 μm or more. The thermal spraying powder is suitable for use in forming a thermal spray coating through high-velocity flame spraying.

BACKGROUND OF THE INVENTION

The present invention relates to a thermal spraying powder that is useful in forming a thermal spray coating through high-velocity flame spraying. The present invention also pertains to a thermal spraying method and a method for forming a thermal spray coating using such a thermal spraying powder.

A technique for providing a thermal spray coating on surfaces of metal components of various types of industrial machines and general machines has been proposed to impart characteristics such as corrosion resistance, wear resistance, and heat resistance to the surfaces. For example, Japanese Patent No. 2969050 and Japanese Patent No. 3155124 each disclose a technique for providing a thermal spray coating made of a chromium-iron based alloy on an inner surface of a cathode compartment of a sodium-sulfur battery to impart resistance to corrosion by sodium polysulfide. In particular, Japanese Patent No. 3155124 discloses a technique for forming a thermal spray coating through plasma spraying of a chromium-iron based alloy powder.

The thermal spray coatings made of a chromium-iron based alloy in Japanese Patent No. 2969050 and Japanese Patent No. 3155124 have low hardness and low wear resistance, and are thus not suitable for use that requires wear resistance. Thermal spray coatings having high wear resistance include a thermal spray coating formed of a cermet powder containing tungsten carbide and cobalt, and a thermal spray coating formed of a cermet powder containing tungsten carbide, cobalt, and chromium. However, thermal spray coatings formed of such cermet powders are very expensive as compared to a thermal spray coating formed of a chromium-iron based alloy. Therefore, under the present circumstance where thermal spray coatings formed of cermet powders are expensive, improving the wear resistance of a thermal spray coating formed of a chromium-iron based alloy is industrially useful.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide a thermal spraying powder that is capable of forming a thermal spray coating made of a chromium-iron based alloy having satisfactory wear resistance. Another objective of the present invention is to provide a thermal spraying method and a method for forming a thermal spray coating that use such a thermal spraying powder.

To achieve the foregoing and other objectives a thermal spraying powder is provided. The thermal spraying powder contains a chromium-iron based alloy powder that includes carbon. The ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 2% or more.

The present invention also provides a method including performing high-velocity flame spraying of the above thermal spraying powder.

The present invention further provides a method for forming a thermal spray coating. The method includes forming a thermal spray coating through high-velocity flame spraying of the above thermal spraying powder.

Other aspects and advantages of the invention will become apparent from the following description illustrating by way of example the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will now be described.

A thermal spraying powder according to this embodiment is formed of a chromium-iron based alloy powder that includes carbon.

If the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is less than 2%, a thermal spray coating formed of the thermal spraying powder has low hardness and unsatisfactory wear resistance. Therefore, in view of obtaining a thermal spray coating having satisfactory wear resistance, the ratio of carbon must be 2% or more. However, although the ratio of carbon is 2% or more, if the ratio of carbon is less than 3%, the wear resistance of the thermal spray coating might be insufficient. Therefore, the ratio of carbon is preferably 3% or more. Meanwhile, if the ratio of carbon is greater than 10%, there is a risk that the wear resistance of the thermal spray coating might be reduced due to embrittlement of the thermal spray coating. Thus, in view of preventing the wear resistance of the thermal spray coating from being reduced due to embrittlement, the ratio of carbon is preferably 10% or less. In the case of the thermal spraying powder of this embodiment, the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is equivalent to the ratio of the mass of carbon in the thermal spraying powder to the total mass of chromium and iron in the thermal spraying powder.

When the content of chromium in the alloy powder is less than 60% by mass, a thermal spray coating formed of the thermal spraying powder might not be so high in hardness. When the hardness of the thermal spray coating is not so high, the wear resistance of the thermal spray coating is not sufficient. Therefore, the content of chromium is preferably 60% by mass or more. Meanwhile, when the content of chromium is greater than 95% by mass, or more specifically greater than 85% by mass, or even more specifically greater than 80% by mass, there is a risk that the adhesion efficiency (thermal spraying yield) might be reduced. Therefore, in view of preventing the adhesion efficiency from being reduced, the content of chromium is preferably 95% by mass or less, and more preferably 85% by mass or less, and most preferably 80% by mass or less. In the case of the thermal spraying powder of this embodiment, the content of chromium in the alloy powder is equivalent to the content of chromium in the thermal spraying powder.

When 10% particle size D₁₀ of the alloy powder is less than 10 μm, or more specifically less than 15 μm, a phenomenon called spitting might occur during thermal spraying. Therefore, in view of preventing spitting from occurring, the 10% particle size D₁₀ is preferably 10 μm or more, and more preferably 15 μm or more. Meanwhile, when the 10% particle size D₁₀ is greater than 25 μm, the adhesion efficiency might be reduced. Therefore, in view of preventing the adhesion efficiency from being reduced, the 10% particle size D₁₀ is preferably 25 μm or less. The 10% particle size D₁₀ of the thermal spraying powder is the size of the particle that is lastly summed up when the volume of particles in the alloy powder is accumulated from particles of the smallest size in ascending order until the accumulated volume reaches 10% of the total volume of all the particles in the alloy powder. The 10% particle size D₁₀ of the alloy powder is measured using, for example, a laser diffraction type of particle size measuring instrument. In the case with the thermal spraying powder of this embodiment, the 10% particle size D₁₀ of the alloy powder is equivalent to the 10% particle size D₁₀ of the thermal spraying powder.

Spitting refers to a phenomenon in which excessively molten thermal spraying powder adheres and accumulates on the inside walls of injection nozzles of a thermal sprayer, which causes contamination of the thermal spray coating due to those deposits falling off during thermal spraying. When spitting occurs, the structure of the thermal spray coating becomes uneven, causing a significant decrease in the quality of the thermal spray coating.

When the 50% particle size D₅₀ of alloy powder is less than 20 μm, there is a risk that spitting may occur. Therefore, in view of preventing spitting from occurring, the 50% particle size D₅₀ is preferably 20 μm or more. Meanwhile, when the 50% particle size D₅₀ is greater than 50 μm, there is a risk that the adhesion efficiency might be reduced. Therefore, in view of preventing the adhesion efficiency from being reduced, the 50% particle size D₅₀ is preferably 50 μm or less. The 50% particle size D₅₀ of the thermal spraying powder is the size of the particle that is lastly summed up when the volume of particles in the alloy powder is accumulated from particles of the smallest size in ascending order until the accumulated volume reaches 50% of the total volume of all the particles in the alloy powder. The 50% particle size D₅₀ of the alloy powder is measured using, for example, a laser diffraction type of particle size measuring instrument. In the case with the thermal spraying powder of this embodiment, the 50% particle size D₅₀ of the alloy powder is equivalent to the 50% particle size D₅₀ of the thermal spraying powder.

The thermal spraying powder is used for forming a thermal spray coating through, for example, high-velocity flame spraying. A thermal spray coating formed through high-velocity flame spraying of the thermal spraying powder has satisfactory wear resistance. A high-velocity flame sprayer that is capable of spraying the thermal spraying powder in a suitable manner includes high-output type high-velocity flame sprayers such as “JP-5000” manufactured by Praxair/TAFA and “Diamond jet (hybrid type)” manufactured by Sulzer Metco.

The preferred embodiment has the following advantages.

A thermal spray coating formed through high-velocity flame spraying of the thermal spraying powder of this embodiment has satisfactory wear resistance. Therefore, the thermal spraying powder is very useful in forming a thermal spray coating that requires wear resistance as a substitute for a cermet powder that contains tungsten carbide and cobalt or a cermet powder that contains tungsten carbide, cobalt, and chromium.

According to high-velocity flame spraying, the thermal spraying powder collides against the surface of the substrate at high speed since the velocity of thermal spraying powder particles injected from the thermal sprayer is high as compared to other spraying methods such as flame spraying and plasma spraying. This produces a thermal spray coating that has high adhesiveness to the substrate and is dense. Furthermore, according to the high-velocity flame spraying, as compared to other spraying methods, the thermal spraying powder is not easily overheated during spraying, which suppresses thermal alteration of the thermal spraying powder. The reasons that the thermal alteration of the thermal spraying powder is suppressed in the case with the high-velocity flame spraying include that the amount of atmosphere that enters the flame is relatively small since the flame, which is a heat source of high-velocity flame spraying, is at high pressure, and that the time period during which the thermal spraying powder stays in the flame is short due to the high velocity of thermal spraying powder particles injected from the thermal sprayer. The wear resistance of the thermal spray coating is improved when the adhesiveness of the thermal spray coating to the substrate is high, the thermal spray coating is dense, or the thermal spray coating does not include the thermal spraying powder that is thermally altered.

The preferred embodiment may be modified as follows.

The chromium-iron based alloy powder may contain components other than carbon, chromium, and iron. However, the total content of carbon, chromium, and iron in the alloy powder is preferably 90% by mass or more, and more preferably 95% by mass or more, and most preferably 98% by mass or more. When the ratio of the mass of silicon in the alloy powder to the total mass of chromium and iron in the alloy powder is greater than 1%, there is a risk that a high-quality thermal spray coating might not be obtained. Therefore, when the alloy powder further contains silicon, the ratio of silicon is preferably 1% or less.

The thermal spraying powder may contain a powder other than chromium-iron based alloy powder. However, the content of the chromium-iron based alloy powder in the thermal spraying powder is preferably 90% by mass or more, and more preferably 95% by mass or more, and most preferably 98% by mass or more.

The thermal spraying powder may be used for forming a thermal spray coating through thermal spraying other than the high-velocity flame spraying.

Next, examples and comparative examples of the present invention will be described.

Thermal spraying powders according to Examples 1 to 4 and Comparative Examples 1 to 3 formed of a chromium-iron based alloy powder were prepared. Details of each thermal spraying powder are shown in Table 1.

Numerical values in the column entitled “Ratio of carbon” in Table 1 represents the ratio of the mass of carbon in the thermal spraying powder to the total mass of chromium and iron in the thermal spraying powder. Numerical values in the column entitled “D₃”, “D₁₀”, “D₅₀”, and “D₉₀” in Table 1 each represents 3% particle size D₃, 10% particle size D₁₀, 50% particle size D₅₀, and 90% particle size D₉₀ of the thermal spraying powder measured using a laser diffraction type of particle size measuring instrument “LA-300” manufactured by HORIBA Ltd. The 3% particle size D₃ of the thermal spraying powder is the size of the particle that is lastly summed up when the volume of particles in the thermal spraying powder is accumulated from particles of the smallest size in ascending order until the accumulated volume reaches 3% of the total volume of all the particles in the thermal spraying powder. The 90% particle size D₉₀ of the thermal spraying powder is the size of the particle that is lastly summed up when the volume of particles in the thermal spraying powder is accumulated from particles of the smallest size in ascending order until the accumulated volume reaches 90% of the total volume of all the particles in the thermal spraying powder.

In Examples 1 to 4 and Comparative Examples 1 and 2, thermal spray coatings having thicknesses of 200 μm were formed on substrates through high-velocity flame spraying of the thermal spraying powders in accordance with first thermal spraying conditions shown in Table 2. In Comparative Example 3, a thermal spray coating having a thickness of 200 μm was formed on a substrate through plasma thermal spraying of the thermal spraying powder in accordance with second thermal spraying conditions shown in Table 2.

Based on occurrence of spitting during thermal spraying, thermal spraying powders of Examples 1 to 4 and Comparative Examples 1 to 3 were evaluated according to a two rank scale: good (1) and poor (2). That is, at a point in time when five minutes have elapsed after starting thermal spraying, if molten thermal spraying powder has adhered to the injection nozzles of the thermal sprayer, the thermal spraying powder was ranked poor, and if molten thermal spraying powder had not adhered to the injection nozzles of the thermal sprayer, the thermal spraying powder was ranked good. The evaluation results are shown in the column entitled “Spitting” in Table 1.

The substrates on which the thermal spray coatings were formed were each cut along a cross-section perpendicular to the surface of the substrate. Then, the cross-sections were subjected to mirror finishing by grinding, washing, and drying. Thereafter, the Vickers hardness of the thermal spray coatings at the cross-sections were measured in accordance with the measuring conditions shown in Table 3 using a Vickers hardness tester “HMV-1” manufactured by Shimadzu Corporation. Based on the measurement results, the hardness of the thermal spray coatings formed of the thermal spraying powders of Examples 1 to 4 and Comparative Examples 1 to 3 were evaluated according to a three rank scale: excellent (1), good (2), and poor (3). That is, when the Vickers hardness (Hv 0.2) was 800 or more, the thermal spraying powder was ranked excellent, when the Vickers hardness (Hv 0.2) was 700 or more and less than 800, the thermal spraying powder was ranked good, and when the Vickers hardness (Hv 0.2) was less than 700, the thermal spraying powder was ranked poor. The measured values of the Vickers hardness and the evaluation results are shown in the column entitled “Hardness” in Table 1.

The thermal spray coatings provided on the substrates were subjected to a dry wear test in reference to JIS H 8682-1. More specifically, the surface of the thermal spray coatings were rubbed 400 times with an abrasive paper (SIC#180) with a load of approximately 31 N (3.15 kgf) using a reciprocating abrasion tester (manufactured by Suga Test Instrument Co., Ltd.). Based on the wear amounts of the thermal spray coatings obtained through the wear test, the wear resistances of the thermal spray coatings formed of the thermal spraying powders of Examples 1 to 4 and Comparative Examples 1 to 3 were evaluated according to a three rank scale: excellent (1), good (2), and poor (3). That is, when the ratio of the wear amount of the thermal spray coating to the wear amount of an authentic sample (SS400 steel plate), which was subjected to the same wear test, was less than 20%, the thermal spraying powder was ranked excellent, when the ratio was 20% or more and less than 30%, the thermal spraying powder was ranked good, and when the ratio was 30% or more, the thermal spraying powder was ranked poor. The ratios of the wear amount of the thermal spray coatings to the wear amount of the authentic sample, and the evaluation results based on the ratios are shown in the column entitled “Wear resistance” in Table 1. TABLE 1 Composition of Wear thermal spraying Hardness resistance powder (mass %) Ratio of Particle size distribution Measured Measured Cr Fe C carbon D₃ D₁₀ D₅₀ D₉₀ Spitting value Evaluation value Evaluation Ex. 1 66.5% 28.3% 5.2% 5.5% 13.8 μm 17.3 μm 31.5 μm 53.9 μm 1 856 1 18% 1 Ex. 2 67.2% 29.6% 3.2% 3.3% 14.3 μm 19.5 μm 28.9 μm 55.2 μm 1 765 2 24% 2 Ex. 3 64.6% 25.6% 9.8%  11% 13.5 μm 15.5 μm 33.5 μm 52.5 μm 1 875 1 27% 2 Ex. 4 66.5% 28.3% 5.2% 5.5% 17.8 μm 24.7 μm 44.5 μm 66.7 μm 1 737 2 28% 2 C. Ex. 1 68.8% 30.9% 0.3% 0.3% 14.6 μm 18.2 μm 35.5 μm 56.5 μm 1 633 3 35% 3 C. Ex. 2 66.5% 28.3% 5.2% 5.5%  7.8 μm  9.3 μm 19.7 μm 38.9 μm 2 711 2 28% 2 C. Ex. 3 66.5% 28.3% 5.2% 5.5% 13.8 μm 17.3 μm 31.5 μm 53.9 μm 2 665 3 48% 3

TABLE 2 First thermal spraying conditions Second thermal spraying conditions Substrate: SS400 steel plate Substrate: SS400 steel plate (7 cm × 5 cm × 2.3 mm, (7 cm × 5 cm × 2.3 mm, degreased and surface degreased and surface roughened with alumina grit #40) roughened with alumina grit #40) Sprayer: “JP-5000” Sprayer: “SG-100” manufactured by Praxair/TAFA manufactured by Praxair/TAFA Oxygen flow rate: 1450 scfh Current: 800 A Kerosene flow rate: 6.0 gph Ar gas pressure: 50 psi Spraying distance: 380 mm He gas pressure: 100 psi Barrel length: 101.6 mm Spraying distance: 120 mm

TABLE 3 Indenter: pyramid made of diamond Angle between opposite faces: 136 degrees Indenter load: 2.0 N (=approximately 0.2 kgf) Hold time after applying load: 15 seconds

As shown in Table 1, in Examples 1 to 4, each of the evaluations for spitting, hardness, and wear resistance is either excellent or good. The results suggest that the thermal spraying powders of Examples 1 to 4 are capable of forming high-quality thermal spray coatings having high hardness and wear resistance without causing spitting. In Example 1, in which the thermal spray coating was formed through high-velocity flame spraying, each of the evaluations for spitting, hardness, and wear resistance is satisfactory as compared to Comparative Example 3 in which the thermal spray coating was formed through plasma spraying. The results suggest that the thermal spraying powder of the present invention is suitable for use in forming the thermal spray coating through high-velocity flame spraying. 

1. A thermal spraying powder comprising a chromium-iron based alloy powder that includes carbon, wherein the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 2% or more.
 2. The thermal spraying powder according to claim 1, wherein the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 3% or more.
 3. The thermal spraying powder according to claim 1, wherein 10% particle size D₁₀ of the alloy powder is 10 μm or more.
 4. The thermal spraying powder according to claim 3, wherein the 10% particle size D₁₀ of the alloy powder is 15 μm or more.
 5. The thermal spraying powder according to claim 1, wherein 50% particle size D₅₀ of the alloy powder is 20 μm or more.
 6. The thermal spraying powder according to claim 1, wherein the content of chromium in the alloy powder is 60% by mass or more.
 7. The thermal spraying powder according to claim 1, wherein the thermal spraying powder is used for forming a thermal spray coating through high-velocity flame spraying.
 8. A method comprising performing high-velocity flame spraying of a thermal spraying powder containing a chromium-iron based alloy powder that includes carbon, wherein the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 2% or more.
 9. A method for forming a thermal spray coating, comprising forming a thermal spray coating through high-velocity flame spraying of a thermal spraying powder containing a chromium-iron based alloy powder that includes carbon, wherein the ratio of the mass of carbon in the alloy powder to the total mass of chromium and iron in the alloy powder is 2% or more. 